Sensor devices and electric field enhancing devices, such as Raman spectroscopic apparatuses, using the electric field enhancing effect of the localized plasmon resonance phenomenon on the surface of a metal have been known. The Raman spectroscopy is a method to obtain a spectrum (Raman spectrum) of Raman-scattered light by separating scattered light which is obtained by applying single wavelength light to a material, and is used to identify materials, etc.
The Raman spectroscopy includes a Raman spectroscopy method called surface enhanced Raman (SERS), which uses optical electric fields enhanced by localized plasmon resonance in order to enhance weak Raman-scattered light (see S. Ghadarghadr et al., “Plasmonic array nanoantennas on layered substrates: modeling and radiation characteristics”, Optics Express, Vol. 17, No. 21, pp. 18556-18570, 2009, hereinafter Non-Patent Literature 1). This uses a principle where, when light is applied to a metal body, in particular, a metal body having nano-order protrusions and recesses on the surface, which is in contact with a material, the optical electric fields are enhanced due to the localized plasmon resonance, and the intensity of Raman-scattered light from a sample in contact with the surface of the metal body is enhanced. The surface enhanced Raman spectroscopy can be performed by using a substrate having an uneven structure of a metal on the surface as a carrier (substrate) for carrying the analyte.
As the substrate having a fine uneven structure of a metal on the surface, a substrate which is formed by forming protrusions and recesses on the surface of a Si substrate and forming a metal film on the surface of the protrusions and recesses is mainly used (see PCT Japanese Publication No. 2006-514286, Japanese Patent No. 4347801, and Japanese Unexamined Patent Publication No. 2006-145230, hereinafter Patent Literature 1, 2, and 3, respectively).
Also, a substrate which is formed by anodizing a surface of an Al substrate to partially form a metal oxide layer (Al2O3), and filling a plurality of fine holes which are naturally formed in the surface of the metal oxide layer during the anodization with a metal has been proposed (see Japanese Unexamined Patent Publication No. 2005-172569, hereinafter Patent Literature 4).
The conventional optical electric field enhancing devices disclosed in Patent Literature 1 to 4 are formed by forming a fine uneven structure on the surface of an opaque substrate, such as Si or Al, and forming a metal film on the surface having the fine uneven structure or filling the recesses with the metal. Patent Literature 4 teaches an example where a transparent substrate, such as a glass substrate, is used; however, the fine uneven structure is made of an opaque material, such as silicon or germanium.
The conventional Raman spectroscopic apparatuses are configured to detect the Raman-scattered light from the sample-surface side. However, in a case where the analyte is a μm-order or larger sample, such as cells, the sample blocks the Raman-scattered light and it is difficult to receive the weak Raman-scattered light at high S/N.
The present inventors have developed, as an optical electric field enhancing device which allows the SERS measurement at good S/N, an optical electric field enhancing device formed by covering the surface of protrusions and recesses of a transparent fine uneven structure called boehmite with a metal film, such as gold. This device allows detecting the Raman-scattered light from the rear surface side of the sample, thereby allowing detecting signal light enhanced by the plasmon-enhanced optical electric fields without being blocked by the analyte (see Japanese Unexamined Patent Publication Nos. 2012-063293 and 2012-063294, hereinafter Patent Literature 5 and 6, respectively).